I'm not sure how common different styles of harpsichord were at different points in history, but many harpsichords have two manuals, multiple courses of strings playing at different octaves, ways of controlling which courses of strings are played by the manuals, and also in many cases a "dog ear coupler" which will cause notes played on the lower manual to also be played on the upper. Thus, the fact that a pitch is played simultaneously on two strings an octave apart may or may not indicate that the performer is pressing two keys.
On the harpsichords I've played, when multiple strings are played by a key, it would be possible to play the notes at distinct times by playing the string absurdly slowly and deliberately, but the plectra are regulated to engage at about the same time and with the same amount of force. It would probably be fairly easy to design or adjust a harpsichord so that one set of jacks had its plectra set to engage sooner than the rest, and operate with less force, so as to allow a performer to either play individual notes using just that set of jacks, using all activated sets, or initially on that set of jacks and then on the other sets after a short delay. I don't know how often instruments would be deliberately designed to work that way, either in Bach's time or in more modern times, but designing them to do so would be fairly easy.